Electron tube with a cathode heat dam



y 5, 1964 J. w. KENDALL, JR 3,132,274

ELECTRON TUBE WITH A CATHODE HEAT DAM Filed Sept. 27, 1961 INVENTOR. JACKSON w. KENDALL JR.

ATTORNEY United States Patent 3,132,274 ELECTRGN TUBE WlTl-I A. CATHQDE HEAT DAM Jackson W. Kendall, J12, Los Altos, Califi, assignor to Eitel-McCullough, Inc, Fran Carlos, Calif a corporation of California Filed Sept. 27, 1961, Ser. No. 141,090 7 Claims. (Cl. 313-47) This invention relates to electron tubes and more particularly to improved heater, cathode and grid assemblies for tubes having cylindrical electrodes. 3

Present day electronic equipment requires many improvements in electron tubes. Among the needed improvements with which this invention is concerned are increased heater efficiency, higher frequency, decreased cost, and greater ability to withstand mechanical shock.

In same respects, this invention advances certain improvernents taught by Patent No. 2,939,988, issued June 7, 1960, to R. D. Culbertson et al., and assigned to the same assignee as this invention. More particularly, said patent to Culbertson et al., discloses an improved cylindrical cathode structure supported directly on a thin metal heat darn. it has been found that when the cathode-to-grid spacing in such a tube is reduced more and more to meet the need for increasingly higher frequencies, an appreciable number of tube failures occur. These failures do not occur until after repeated operation, but they occur short of the expected full life of the tube. It has been determined according to this invention that such failures are caused by a slight outward flaring of the cathode cylinder at the end where it is joined to the heat dam. When the cathode is surrounded by an extremely close spaced grid, even a slight flaring brings the cathode into detrimentally close proximity to the grid or even into shorting contact with the grid. It was finally determined according to the invention that the flaring was caused by the fact that the best metals for the cathode cylinder and heat darn, such as nickel and Kovar, respectively, have substantially dilierent coefiicients of expansion.

Detection of the cause of tube failures was complicated by the fact that the failures did not occur until after substantial operation of the tubes. Even after it was determined that flaring was the cause or failure and even after it was noted that the cathode cylinder and heat dam had different coefiicients of expansion, the cause of the flaring was still not readily apparent. One reason it was not apparent is that one would expect that if differing coefficients of expansion were the cause, then the flaring should occure the first time the tube was brought up to operating temperature. However, no flaring was detectable after the normal test operations or even after substantial actual use of the tubes. Even more confusing was the fact that Kovar, the heat dam material, has a substantially lower coefiicient of expansion than nickel, the cathode material. Thus, it would be expected that if the difference in coefiicient of expansion had any effect at all, the effect should be directly opposite from the observed result. More specifically it would be expected that since the Kovar cylinder has a lower coefficient of expansion, it would restrain the expansion of the end of the nickel cylinder to which it is attached and result in an inward flaring of the end of the nickel cylinder, rather than the observed outward flaring.

It was finally reasoned according to the invention that the thin Kovar and nickel cylinders are weaker in expansion than in compression so that each time the structure is heated the expanding nickel is able to overpower the Kovar. Thus, when the structure is heated the first time, the expanding nickel cylinder stretches the Kovar cylinder a very small amount beyond the diameter the Kovar would assume under its own expansion force. Then when the Kovar cools down it shrinks a specific "ice,

amount. This specific amount is of course the amount required to reduce the Kovar to its original diameter when it shrinks from the expanded diameter to which it would be driven by its own expansion forces. However, the nickel end of the Kovar cylinder has been stretched beyond the diameter to which it would be expanded by its own forces; so that when it shrinks said specific amount, it returns to a diameter slightly larger than its original diameter. Of course, the nickel tends to return to its original diameter, but this shrinking force is unable to overpower the Kovar cylinder in compression. Thus when the structure cools down from its first heating, the

l lovar-nickel joint area has a minutely larger diameter than the remainder of the Kovar and nickel cylinders. The second time the structure is heated to operating temperature, the nickel expands the same specific amount it expanded on the first heating but this time it starts from a larger initial diameter and therefore grows to a larger expanded diameter, forcing the Kovar to expand with it. When the Kovar cools down from this second heating, it contracts the same specific amount it contracted after the first heating, but since it starts with a larger expanded diameter, it ends the second cooling with a slightly larger diameter than it ended the first cooling. The effect of any one or even several such heat cyclings is minute, but when the process is repeated many times the cumulative result is appreciable. The preceding explanation is slightly oversimplified because it does not take into account the elasticity or residual stresses in the metals. F or example, if the residual stresses in the overexpanded Kovar and the undercontracted nickel were not at all relieved, then the Kovar would tend to return to its original diameter after its first overexpanded heat'mg, and the undercontracted nickel would tend to expand on second heating to the same expanded diameter it reached on first heating. Apparently, however, the temperature and time periods involved are suflicient to cause enough annealing to permit the described result.

It is also note according to the invention that the same type of analysis shows that a growing outward flaring of the cathode cylinder will result regardless of whether the heat dam cylinder has a lower'or higher coefficient of expansion than the cathode cylinder. Thus, if the heat dam has a higher coefiicient of expansion than the cathode, the heat dam will overexpand the cathode upon heating, and the cathode will prevent full contraction of the heat dam upon cooling.

Accordingly, it is an object of this invention to provide an improved cathode cylinder and heat dam structure which does not cause flaring and at the same time provides a strong structure permitting use of optimum materials for the cathode and the heat dam; that is materials having different coefiicients of expansion. 7

Another object of the invention is to provide an improved electron tube wherein an electrically conductive cylinder is closely surrounded by and insulated from another electrically conductive member and wherein it is necessary to support said cylinder by another cylinder having a different coefiicient of expansion.

The previously defined patent to Culbertson et al. also discloses an improved heater package involving a heater wire formed in a cylindrical outline and supported between two ceramic end disks. It is another object of this invention to provide a heater package which employs the benefits of the Culbertson et al. heater and which is improved from the standpoints of heater efliciency and cost. An associated object of this invention is to employ a double-function structure which both improves the heater package and solves the cathode flaring problem.

A further object or" the invention is to provide an improved cathode-grid assembly which extremely rugged al and able to withstand severe mechanical shock and vibration.

Another object of the invention is to provide an electron tube with a triple function structure which not only solves the flaring problem but also contributes to solution of the problems of improving heater efficiency, cost, and structural rigidity.

By way of brief description, the invention applies the previously explained reasoning and inserts an intermediate member between the cathode cylinder and heat dam. The intermediate member has the same coetficient of expansion as the cathode cylinder in order to solve the flaring problem and is shaped to cooperate with other iembers in providing the improvement of better heater efficiency, cost reduction and cathode-grid stability.

Other objects and features of advantage will be apparent from the following detailed description read in conjunction with the accompanying drawings in which:

FIGURE 1 is a cross-sectional view of an electron tube embodying the invention, and

FIGURE 2 is a cross-sectional view of a portion of the tube which is particularly important in the invention.

Referring in more detail to the drawing, FIGURE 1 discloses in cross-section a generally cylindrical tetrode electron tube comprising a cathode l, a control grid 2, a screen grid 3 and an anode d.

The cathode comprises a metal can having a cylindrical side wall 8 and an indented upper end wall 9.

The cathode is preferably made of nickel and in a specific embodiment a as a wall thickness of .010 inch. The outer surface of wall 9 is coated with a conventional electron emissive layer of the oxide type. Flaps 10 are cut out of the periphery of end wall 9 and are bent over a thin metal disk 11 which forms part of a thermal insulating structure comprising alternate metal disks ll and corrugated metal sheets 32. The lisks 11 and sheets 12 are made of extremely thin metal such as Kovar or molybdenurn. The heater is formed by a wire M bent to lie in a generally cylindrical shape and having leg portion 15 extending lengthwise of the cylindrical shape and joined at the ends by connecting portions 16. The heater is held in place by means of ceramic rings or retaining members 18 and 19. The retaining members are provided with grooves and 21, respectively, which receive the heater wire connecting portions 16 and keep the heater in its cylindrical shape. Since the retaining members 18 and 19 are ceramic it is not necessary to provide an insulating coating on the heater wire. Although the described arrangement is preferred, it is of course possible to provide the heater wire with an insulating coating and then use retaining members made of metal.

In order to hold the retaining members 1% and 19 fixed in axial spacing from each other and thus in retaining contact with the heater wire, the upper retaining anember 19 is in abutting relation with the upper end 9 0n the cathode cylinder. The lower retaining member 18 is held in place by a three-function member 24 which comprises a generally cylindrical side wall 25 having a radially extending abutment portion 26 with a downturned rim portion 27. In addition, member 24 includes a bottom portion 28 having a central aperture 29. In order not to crowd the showing in FIGURE 1, member 24 is identified in detail in FIGURE 2. The member 24 is made of a material having the same coefiicient of expansion as the material of cathode 1. The easiest way to achieve the match in coefficient of expansion is to use the same materials. Therefore, since the preferred material :for cathode 1 is nickel, the preferred material for member 24 is also nickel. The selection of wall thickness for member 24 is necessarily a compromise between making it thick enough for required strength and thin enough that it will not present an appreciable heat dissipating mass. In the specific embodiment a thickness of .010 inch has been found satisfactory. The rim 27 on member 24 is attached to the lower end of the cathode cylinder 8 as by spot welding.

The description of the entire tube structure will be completed next, and thereafter the importance of the various features of member 24 in relation to the associated parts will be described in detail.

Thermal insulation for the bottom of the heater enclosure is provided in the form of alternate layers of thin metal disks 32 and corrugated sheets 33 similar to disks 11 and sheets 12. The heater has one end lead 34 which passes through a ceramic insulator 35 received in suitable apertures in the thermal insulation and the bottom of member 24. The other end lead of the heater is shown at 36 and passes through suitable apertures in members 18 and 25. Since one end of the heater is preferably at cathode potentim in the specific tube embodiment, there is no need for an insulator around lead 36.

The heater-cathode structure thus far described is supported on a heat dam 39 which consists of an extremely thin-wall metal cylinder. The thermal effectiveness of the heat dam depends on the thermal conductivity coefficient of the material and on the wall thickness. Thus, for thermal purposes the heat dam should be made of a matcrial having a very low coefiicient of thermd conductivity and should have minimum wall thickness. In addition the heat dam must be strong enough to rigidly support the heater-cathode structure. One material well suited to fill these anomalous requirements is Kovar. Other materials such as molybdenum-nickel eutectic type alloys can be used, but in any case the best materials for the heat dam have a different coefficient of expansion than the best materials for the cathode cylinder. Thus, when the heat dam is connected directly to the cathode cylinder, the different coeilicients of expansion raise the previously described flaring problem discovered and solved by this invention. In the preferred embodiment the heat dam is made of Kovar having a wall thickness of .002 inch. Obviously, the thickness of the heat dam is much exaggerated in FIGURE 1 in order to be visible in the drawing. Heat dam 39 is attached at its lower end to a support cylinder 40 preferably made of Kovar or nickel having a wall thickness of .010 inch. The connections at both ends of the heat dam are preferably made by spot welding.

The lower end of member 4-0 is attached as by spot welding to a metal support cone 41 preferably made of .020 inch nickel. The heater lead 34 is connected to a metal lead strip 42 which is in turn connected to an inverted metal shielding cup 43. The shielding cup is attached to a metal end closure member on a hollow metal post 45 which carries an indexing pin 46. The post and pin arrangement are socketing features described in detail and claimed in a Patent No. 2,977,494, issued March 28, 1961, to F. C. Iohnstone et al. and assigned to the assignee of this invention. The lower end of post 45 is supported by a metal end disk 47, and the upper end of the post is supported by a conical portion 48 extending down from closure member 44. The heater lead 36 is firmly connected to cathode potential by means of a metal strip 49 attached to th support cylinder 40.

The control grid 2 and screen grid 3 are conventional cage type structures made of circumferentially spaced wire rods or bars 52 and 53, respectively. Rigidity is added by longitudinally spaced reinforcing rings 54 and 55, respectively. The upper ends of the longitudinal members 52 and 53 are connected to cup-shaped metal caps 56 and 57, respectively. The lower end of grid 2 is supported by a one-piece metal support cylinder 58, and the lower end of grid 3 is supported by a three-piece metal support cylinder 59.

The cathode-to-grid and grid-to-grid alignment is accurately achieved and rigidly maintained by means of a ceramic center pin 62. The pin is received in the center aperture 29 in member 24 and in coaxial apertures in members 9, 56 and 57. The surface of the pin is preferably metalized at 63 and 64, and a brazed connection is made between these metalized areas and cap members 56 and 57, respectively. In the preferred embodiment the pin has a close sliding fit in the apertures in members 9 and 24.

The anode 4 is a metal cup-shaped member provided with a conventional exhaust tubulation 67 covered in the usual fashion by protective cap members 68 and 69. The anode structure is completed in conventional manner by radially extending cooling fins '70.

The tube envelope or side wall below the anode comprises ceramic rings 7277, which rings are metalized at their ends and brazed to the previously described support members 47, 48, 41, 58 and 59. Each of these support members except 47 is provided with circumferentially spaced terminal tabs projecting outwardly from the ceramic rings, as shown at the left in FIGURE 1. The final seal for the tube is made by heliarcing the outer edges of a sealing ring 78 brazed to the ceramic wall portion and a sealing ring 79 brazed to the anode.

Returning now to the three-fold function of member 24, it will be noted that the portions 25, 26 and 27 form a generally cylindrical support member interposed between the cathode cylinder 8 and the heat dam 39. It will also be recalled that the member 24 is required to have the same coefficient of expansion as the cathode cylinder. Thus, the use of member 24 provides an eflicient solution to the complicated flaring problem which eventually results when the cathode cylinder is directly connected to the heat dam. Even if some flaring were to occur at the connection between member 24 and heat dam 39, the location of the connection is radially spaced so far from the nearest grid 2 that no damage could result. Further, the presence of the disk-shaped portion 28 will tend to exert enough contracting force when the structure is cooled to overpower the Kovar member 39 even in compression and return the structure to its original size after each heating.

Another function of member 24 is to provide an extremely rigid connection between the cathode 1 and the grids 2, 3. This function is accomplished by the fact that the bottom portion 28 serves as a fourth contact point for support pin 62. The cathode end wall 9 would supply some contact for pin 62, but by itself wall 9 is quite weak. However walls 9 and 28 together supply a very strong socket for pin 62 because of the long axial spacing between walls 9 and 28.

A third function of member 24 is to improve the thermal efiiciency and reduce the cost of the heater-cathode package. More specifically, the member 24 serves the several purposes of holding the abutment member 18 tight against the heater 14; providing a bottom closure for the heatercathode package; and providing a receiving well for the thermal insulation 32, 33. Since all of these purposes are served by a simple, low-mass member, the cost and heat dissipation of a number of single purpose members may thus be eliminated.

Having thus described the invention what is claimed as new and desired to be secured by Letters Patent is:

1. An electron tube having an envelope enclosing cylindrical electrodes including a cathode, said cathode comprising a cylinder having an electron emissive outer surface, another electrode closely surrounding said cathode, support means for said cylinder comprising a generally cylindrical support member having the same coeficient of expansion as said cathode cylinder, said support member having a downturned rim nested inside the lower end of said cathode cylinder and joined thereto, a generally cylindrical heat darn joined to said support member and having a coefficient of expansion substantially different from said cathode cylinder, and means supporting said heat dam on said envelope.

2. An electron tube having cylindrical electrodes including a cathode, said cathod comprising a cylinder having an electron emissive surface, a heater for said cathode comprising heater wire bent to lie in a generally cylindrical shape with leg portions extending generally lengthwise of said cylindrical shape and joined at the ends by'connecting portions, a retaining member at each end of said cylindrically shaped heater wire, each of said retaining members having an annular recess in which said connecting portions of the heater wire are seated, an end member attached to one end of said cathode cylinder and engaging one of said retaining members, and an end member at tached to the other end of said cathode cylinder and engaging the other of said retaining members, said end members holding said retaining members against said heater wire.

3. An electron tube having an envelope enclosing cylindrical electrodes including a cathode, said cathode comprising a cylinder having an electron emissive outer surface, another electrode closely adjacent the outer surface of said cathode cylinder, support means for said cathode cylinder comprising a support member having the same coefiicient of expansion as said cathode cylinder; said support member comprising a cylindrical side wall portion, an outwardly extending flange portion at one end of said side wall, a downturned rim portion on the flange portion, and an end portion across the other end of said side wall; said rim portion being nested in and joined to one end of said cathode cylinder, a cylindrical heat dam joined to the outside of said side wall portion and having a coeflicient of expansion substantially different from said cathode cylinder, and means supporting said heat dam on said envelope.

4. An electron tube having an envelope enclosing cylindrical electrodes including acathode, said cathode comprising a cylinder having an electron emissive outer surface, another electrode closely adjacent the outer surface of said cathode cylinder, support means for said cathode cylinder comprising a support member having the same coefficient of expansion as said cathode cylinder; said support member comprising a cylindrical side wall portion, an outwardly extending flange portion at one end of said side wall, a downturned rim portion on the flange portion, and an end portion across the other end of said side wall; said rim portion being nested in and joined to the lower end of said cathode cylinder, a cylindrical heat dam joined to the outside of said side wall portion and having a coeflicient of expansion substantially different from said cathode cylinder, means supporting said heat dam on said envelope, a heater inside said cathode comprising heater wire bent to lie in a generally cylindrical shape with leg portions extending generally lengthwise of said cylindrical shape and joined at the ends by connecting portions, a dielectric retaining member at each end of said cylindrically shaped heater wire and engaging said connecting portions, and one of said retaining members being in axial abutment with said flange portion of said support member.

5. An electron tube having an envelope enclosing cylindrical electrodes including a cathode, said cathode comprising a cylinder having an electron emissive outer surface, another electrode closely adjacent the outer surface of said cathode cylinder, support means for said cathode cylinder comprising a support member having the same coeflicient of expansion as said cathode cylinder; said support member comprising a cylindrical side wall portion, an outwardly extending flange portion at one end of said side Wall, a downturned rim portion on the flange portion, and an end portion across the other end of said side wall; said rim portion being nested in and joined to one end of said cathode cylinder, a cylindrical heat dam joined to the outside of said side wall portion and having a coefiicient of expansion substantially different from said cathode cylinder, means supporting said heat dam on said envelope, a heater inside said cathode comprising heater wire bent to lie in a generally,

cylindrical shape with leg portions extending generally lengthwise of said cylindrical shape and joined at the ends by connecting portions, a dielectric retaining member at each end of said cylindrically shaped heater wire and engaging said connecting portions, one of said retaining members being in axial abutment with said flange portion of said support member, a support pin extending coaxially through said cathode cylinder, one end of said pin being connected to said other electrode, and the other end of said pin being in radial engagement with said end portion of the support member.

6. An electron tube having an envelope enclosing cylindrical electrodes including a cathode, said cathode comprising a cylinder having an electron emissive outer surface, another electrode closely adjacent the outer surface of said cathode cylinder, support means for said cathode cylinder comprising a support member having the same coefficient of expansion as said cathode cylinder; said support member comprising a cylindrical side wall portion, an outwardly extending fiange portion at one end of said side wall, a downturned rim portion on the flange portion, and an end portion across the other end of said side wall; said rim portion being nested in and joined to one end of said cathode cylinder, a cylindrical heat darn joined to the outside of said side wall portion and having a coefficient of expansion substantially different from said cathode cylinder, means supporting said heat dam on said envelope, a heater inside said cathode comprising heater Wire bent to lie in a generally cylindrical shape with leg portions extending generally lengthwise of said cylindrical shape and joined at the ends by connecting portions, a dielectric retaining member at each end of said cylindrically shaped heater Wire and engaging said conecting portions, one of said retaining members being in axial abutment with said flange portion of said support member, said cylindrical side wall and end portions of the support member forming a well, and thermal insulation positioned in said well.

7. An electron tube having an envelope enclosing cylindrical electrodes including a cathode, said cathode comprising a cylinder having an electron emissive outer surface, another elcctrode closely adjacent the outer surface of said cathode cylinder, support means for said cathode cylinder comprising a support member having the same coefiicient of expansion as said cathode cylinder; said support member comprising a cylindrical side wall portion, an outwardly extending flange portion at one end of said side wall, a downturned rim portion on the flange portion, and an end portion across the other end of said side Wall; said rim portion being nested in and joined to one end of said cathode cylinder, a cylindrical eat dam joined to the outside of said side Wall portion and having a coeificient of expansion substantially different from said cathode cylinder, means supporting said heat dam on said envelope, a heater inside said cathode comprising heater wire bent to lie in a generally cylindrical shape with leg portions extending generally lengthwise of said cylindrical shape and joined at the ends by connecting portions, a dielectric retaining member at each end of said cylindrically shaped heater wire and engaging said connecting portions, one of said retaining members being in axial abutment with said flange portion of said support member, said cylindrical side wall and end portions of the support member forming a well, thermal insulation positioned in said well, a support pin extending through said cathode cylinder, one end of said pin being connected to said other electrode, and the other end of said pin being engaged by said end portion of the support member to prevent radial movement of said pin relative to the end portion.

References Cited in the file of this patent UNITED STATES PATENTS 2,471,424 Glauber May 31, 1949 2,582,684 Drieschman et al. Jan, 15, 1952 2,939,988 Culbertson et al. June 7, 1960 3,028,518 Foote et al. Apr. 3, 1962 

1. AN ELECTRON TUBE HAVING AN ENVELOPE ENCLOSING CYLINDRICAL ELECTRODES INCLUDING A CATHODE, SAID CATHODE COMPRISING A CYLINDER HAVING AN ELECTRON EMISSIVE OUTER SURFACE, ANOTHER ELECTRODE CLOSELY SURROUNDING SAID CATHODE, SUPPORT MEANS FOR SAID CYLINDER COMPRISING A GENERALLY CYLINDRICAL SUPPORT MEMBER HAVING THE SAME COEFFICIENT OF EXPANSION AS SAID CATHODE CYLINDER, SAID SUPPORT MEMBER HAVING A DOWNTURNED RIM NESTED INSIDE THE LOWER END OF SAID CATHODE CYLINDER AND JOINED THERETO, A GENERALLY CYLINDRICAL HEAT DAM JOINED TO SAID SUPPORT MEMBER AND HAVING A COEFFICIENT OF EXPANSION SUBSTANTIALLY DIFFERENT FROM SAID CATHODE CYLINDER, AND MEANS SUPPORTING SAID HEAT DAM ON SAID ENVELOPE. 